1. Field of The Invention
The present invention relates generally to an intake- and/or exhaust-valve timing control system for an internal combustion engine. More particularly, the invention is directed to an intake-and/or exhaust-valve timing control system which serves to modify the intake-and/or exhaust-valve timing quickly in response to variation in engine operation parameters.
2. Description of The Prior Art
U.S. Pat. No. 4,535,731 assigned to Alfa Romeo Auto S.p.A. and U.S. Pat. No. 5,088,456 assigned to the same assignee of this application disclose conventional valve timing control systems for internal combustion engines. The latter system represents an improvement on the former and includes a sprocket mechanically connected to a crankshaft of an internal combustion engine through a timing chain, a camshaft disposing cams for opening and closing intake valves according to rotation of the sprocket, an intermdiate cylindrical gear element engaging between the sprocket and the camshaft, and a driving mechanism serving to vary valve timing. The driving mechanism is responsive to variation in operating parameters of the engine to displace the intermediate cylindrical gear element in an axial direction for modifying a phase angle between the sprocket and the camshaft, advancing or retarding the valve timing of the intake valves.
The driving mechanism includes first and second pressure chambers and a solenoid operated actuator for selectively regulating hydraulic pressures supplied to the first and second pressure chambers. When an engine load is increased to a preselected intermediate level, the hydraulic pressure in the first pressure chamber is elevated and then acts on a movable member provided in the first pressure chamber to be displaced for thrusting the intermediate cylindrical gear element from an initial position to an intermediate position so that the valve timing is changed to timing suitable for the intermediate engine load level. When the engine load is further increased, a hydraulic line is changed from the first pressure chamber to the second pressure chamber to increase the hydraulic pressure in the second pressure chamber while the hydraulic pressure in the first pressure chamber is decreased. The elevated hydraulic pressure in the second pressure chamber acts on both the movable member and the intermediate cylindrical gear element so that the movable member is moved in a direction opposite displacement of the intermediate cylindrical gear element. In other words, part of the elevated hydraulic pressure in the second pressure chamber is consumed in displacing the movable member. Thus, the internal pressure in the second pressure chamber required for displacing the intermediate cylindrical gear element is somewhat reduced momentarily, resulting in a response rate for varying the valve timing being delayed.